1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image of a plurality of colors on a recording medium and an image processing method therefor.
2. Description of the Related Art
Conventionally, electrophotography has been known as an image recording method used by a color image-forming apparatus, such as a color printer or a color copying machine. In electrophotography, an electrostatic latent image is formed on a photosensitive drum using a laser beam, and then developed by an electrically charged color material (hereinafter referred to as toner). Further, image recording is performed by transferring the developed toner image onto a transfer sheet and fixing the toner image on the sheet.
In recent years, to increase the image forming speed of the color image-forming apparatus using electrophotography, an increasing number of tandem color image-forming apparatuses have come into use which are each provided with developing devices and photosensitive drums both corresponding in number to the number of colors of toner, for sequentially transferring images of different colors onto an image conveyor belt or a recording medium.
It is known that in the color image-forming apparatus of the tandem type, there are a plurality of factors which cause misregistration, and there have been proposed various measures against the factors.
The factors include non-uniformity of a lens of a deflection scanner or misalignment of the mounting positions of the lens in the deflection scanner, and assembly misalignment of the deflection scanner to a color image-forming apparatus. Such misalignments cause scanning lines to be inclined or curved, and the curvature (hereinafter referred to as ‘the profile’) of each scanning line differs from color to color, which results in misregistration.
The profile differs between image forming apparatuses, i.e. recording engines, and further between colors. FIGS. 17A to 17D are graphs showing examples of profiles characteristic of a conventional image forming apparatus. That is, FIGS. 17A to 17D shows profile characteristics of respective colors, cyan (C), magenta (M), yellow (Y), and black (K). The vertical axis of each graph represents the amount of displacement in the sub scanning direction with respect to an ideal characteristic in the image forming apparatus, while the horizontal axis represents the position in the main scanning direction in the image forming apparatus.
Lines 201, 203, 205, and 207 linearly extending in the main scanning direction represent an ideal characteristic without a curve. On the other hand, curved lines 202, 204, 206, and 208 represent color-specific profile characteristics. More specifically, the profile characteristic of cyan is represented by the line 202. The profile characteristic of magenta is represented by the line 204. The profile characteristic of yellow is represented by the line 206. The profile characteristic of black is represented by the line 208.
As can be understood from FIGS. 17A to 17D, the lines 201, 203, 205, and 207 curve differently, and the differences in curve shape between the colors appear as misregistration in image data after fixing.
As a measure to cope with the misregistration, there has been proposed a method in Japanese Patent Laid-Open Publication No. 2002-116394, in which during the process of assembly of a deflection scanner, the degree of curvature of a scanning line is measured using an optical sensor, and is adjusted by mechanically rotating a lens, and then the lens is fixed by an adhesive.
Further, in Japanese Patent Laid-Open Publication No. 2003-241131, there has been disclosed a method in which during the process of mounting a deflection scanner in the body of an image forming apparatus, the degree of inclination of a scanning line is measured using an optical sensor, and is adjusted by mechanically tilting the deflection scanner, and then the deflection scanner is assembled to the body of the image forming apparatus.
Furthermore, in Japanese Patent Laid-Open Publication No. 2004-170755, there has been proposed a method in which after the inclination and curvature of a scanning line are measured using an optical sensor, bitmap image data is corrected such that the inclination and curvature of the scanning line can be cancelled out, and then the corrected image is formed. In this method, correction is performed by processing image data using software, so that it is possible to dispense with members for mechanical adjustment, and no adjustment process is required to be carried out during assembly. This makes it possible to reduce the size of a color image-forming apparatus and cope with misregistration at a lower cost than by the methods disclosed in Japanese Patent Laid-Open Publications No. 2002-116394 and No. 2003-241131.
The misregistration correction by data processing using software is categorized into pixel-by-pixel correction and less-than-pixel correction. FIGS. 18A to 18C are diagrams useful in explaining the pixel-by-pixel correction. In the pixel-by-pixel correction, pixels are offset in the sub scanning direction on a pixel-by-pixel basis according to the amount of correction of the inclination and curvature of the scanning line. It should be noted that a position from which pixels are offset is hereinafter referred to as “a pixel shift point”. In FIGS. 18A to 18C, each of positions P1 to P5 corresponds to a pixel shift point.
FIGS. 19A to 19E are diagrams useful in explaining the less-than-pixel correction. In the less-than pixel correction, each of the gradation values of bitmap image data is adjusted by adjacent pixels in the sub scanning direction. More specifically, when the scanning line is curved upward due to a profile characteristic shown in FIG. 19A, bitmap image data yet to be subjected to gradation correction is processed in a manner shifted in a direction opposite to a direction shown by the profile characteristic with respect to the sub scanning direction. The correction in an amount of less than one pixel (less-than-pixel correction) is performed by this method, whereby unnatural steps produced by the pixel-by-pixel correction at a pixel shift point as a boundary can be eliminated to thereby smooth the image.
However, in the above-described conventional image forming apparatus, even after correction is performed based on pixel shift points for each color, there is a fear that color moire might cause degradation of image quality in an image visualized after image formation in a case where color-specific pixel shift points are close to each other.